100
I. Ivanenko et al.
In recent years, attention has been directed to graphene-like structures. This is
due to the fact that in 2004 researchers from Manchester university (Novoselov
and Gaym) [1] discovered a simple method of producing graphene and succeeded
in revealing its new properties. Chalcogenides of transition metals fall under the
category of graphene-like structures, the quantity of its investigations continuously
increase.
Chalcogenides of transition are widely investigated and considered for use
in many branches, from producing of new oils to optoelectronics. Given that
chalcogenides of transition metals are semiconductors and chemically inert, there
is interest in them as photocatalysts.
Present-day literature contains conflicting data about the mechanism of photocatalytic reactions. Moreover, it is the titanium photocatalysts that are mainly
investigated. The mechanisms of reactions on the surface of other photocatalysts
are not investigated enough.
7.1.2 The Mechanisms of Photocatalytic Reactions
One of the well-known photocatalysts in present times is semiconductor titanium
(IV) oxide TiO 2 with crystal modification of anatase. It shows high photocatalytic
activity, chemical stability, low toxicity, and comparatively low price. The photocatalytic properties of TiO 2 are caused features of its electron structure, namely,
existing of the valence band and the conduction band in it. Therefore, the most
appropriate would be to consider the mechanism of reaction under light quanta on
titanium (IV) oxide.
At absorption of light quanta, at titanium (IV) oxide irradiation by volume of
particle, the electron vacation h + is formed and free electron e − by the route shown
in reaction (7.1):
TiO 2 + hν → TiO 2
e – + h
+
.
(7.1)
The vacation h + and free electron recombine or transit in the body of semiconductor, partly localizing on defective structural centers of its lattice.
As is known, in semiconductor structures electrons can be in two states: free
and bound [2]. In the first case, electrons move around the lattice of titanium (IV)
oxide, formed by Ti 4+ cations and ± 2− anions. In the second case, electrons, mainly,
bind with any ion of the lattice and take part in formation of chemical bonds. It is
necessary to expend energy equal to the energy of bandgap (for TiO 2 it is 3.2 eV)
for transition of electron from bound state into free one [3].
Holes are formed on TiO 2 surface reactive particles. The mechanism of hole
formation is not yet clear. Hole reacts with water, or with some adsorbed organic,
sometimes inorganic, compound according to reactions (7.2 and 7.3):
I. Ivanenko et al.
In recent years, attention has been directed to graphene-like structures. This is
due to the fact that in 2004 researchers from Manchester university (Novoselov
and Gaym) [1] discovered a simple method of producing graphene and succeeded
in revealing its new properties. Chalcogenides of transition metals fall under the
category of graphene-like structures, the quantity of its investigations continuously
increase.
Chalcogenides of transition are widely investigated and considered for use
in many branches, from producing of new oils to optoelectronics. Given that
chalcogenides of transition metals are semiconductors and chemically inert, there
is interest in them as photocatalysts.
Present-day literature contains conflicting data about the mechanism of photocatalytic reactions. Moreover, it is the titanium photocatalysts that are mainly
investigated. The mechanisms of reactions on the surface of other photocatalysts
are not investigated enough.
7.1.2 The Mechanisms of Photocatalytic Reactions
One of the well-known photocatalysts in present times is semiconductor titanium
(IV) oxide TiO 2 with crystal modification of anatase. It shows high photocatalytic
activity, chemical stability, low toxicity, and comparatively low price. The photocatalytic properties of TiO 2 are caused features of its electron structure, namely,
existing of the valence band and the conduction band in it. Therefore, the most
appropriate would be to consider the mechanism of reaction under light quanta on
titanium (IV) oxide.
At absorption of light quanta, at titanium (IV) oxide irradiation by volume of
particle, the electron vacation h + is formed and free electron e − by the route shown
in reaction (7.1):
TiO 2 + hν → TiO 2
e – + h
+
.
(7.1)
The vacation h + and free electron recombine or transit in the body of semiconductor, partly localizing on defective structural centers of its lattice.
As is known, in semiconductor structures electrons can be in two states: free
and bound [2]. In the first case, electrons move around the lattice of titanium (IV)
oxide, formed by Ti 4+ cations and ± 2− anions. In the second case, electrons, mainly,
bind with any ion of the lattice and take part in formation of chemical bonds. It is
necessary to expend energy equal to the energy of bandgap (for TiO 2 it is 3.2 eV)
for transition of electron from bound state into free one [3].
Holes are formed on TiO 2 surface reactive particles. The mechanism of hole
formation is not yet clear. Hole reacts with water, or with some adsorbed organic,
sometimes inorganic, compound according to reactions (7.2 and 7.3):